PG
By Biblical standards, Proverbs is extremely tame—there's no mass slaughter of any kind, no actual sex (despite a long discussion of adultery), no one getting high off of inhalants or whatever (not that that happens in the Bible, come to think of it). But it gets a mild "PG" rating because it has broadly "adult" themes—a discussion of adultery seems sort of out place in a "G" rated movie, don't you think?